CVD apparatus is conventionally used to form various thin films in a semiconductor integrated circuit. Such CVD apparatus can form thin films such as SiO.sub.2, Si.sub.3 N.sub.4, Si or the like with high purity and high quality. In the reaction process of forming a thin film, a reaction vessel in which semiconductor substrates are arranged can be heated to a high temperature condition of 500.degree. to 1000.degree. C. Raw material to be deposited can be supplied through the vessel in the form of gaseous constituents so that gaseous molecules are thermally disassociated and combined in the gas and on a surface of the substrates so as to form a thin film.
A plasma-enhanced CVD apparatus utilizes a plasma reaction to create a reaction similar to that of the above-described CVD apparatus, but at a relatively low temperature in order to form a thin film. The plasma CVD apparatus includes a process chamber consisting of a plasma generating chamber which may be separate from or part of a reaction chamber, a gas introduction system, and an exhaust system. For example, such a plasma-enhanced CVD apparatus is disclosed in U.S. Pat. No. 4,401,504 and commonly-owned U.S. Pat. No. 5,200,232. Plasma is generated in such an apparatus by a high density microwave discharge through electron-cyclotron resonance (ECR). A substrate table is provided in the reaction chamber, and plasma generated in the plasma formation chamber passes through a plasma extracting orifice so as to form a plasma stream in the reaction chamber. The substrate table may include a radiofrequency (rf) biasing component to apply an rf bias to the substrate and a cooling mechanism in order to prevent a rise in temperature of the substrate due to the plasma action.
A plasma apparatus using high density ECR for various processes such as deposition, etching and sputtering to manufacture semiconductor components is disclosed in U.S. Pat. No. 4,902,934. Such a plasma apparatus includes an electrostatic chuck (ESC) for holding a substrate (such as a silicon wafer) in good thermal contact and in a vertical orientation. The chuck can also be provided with cooling and heating capabilities. In general, such reaction chambers can be operated under vacuum conditions, and the plasma generation chamber can be enclosed by walls which are water-cooled. Other types of reactors in which deposition can be carried out include parallel plate reactors and high density transformer coupled plasma (TCP.TM.), also called inductively coupled plasma (ICP), reactors of the type disclosed in commonly owned U.S. Pat. Nos. 4,340,462 and 4,948,458.
Electrostatic chucking devices are disclosed in U.S. Pat. Nos. 3,993,509; 4,184,188; and 4,384,918. With such systems, a wafer substrate is typically located on a dielectric layer, and the wafer supporting surface of such electrostatic chucking arrangements can be larger or smaller than the wafer substrate supported thereon. The electrostatic voltage and rf bias are applied to an electrode buried within a dielectric layer and proximate to the wafer/substrate contact surface.
In semiconductor processing, devices are being built with smaller wiring pitches and larger interconnect resistances. In order to reduce delays in critical speed paths, it has been proposed to embed low dielectric constant material between adjacent metal lines or lower the dielectric constant of the intermetal dielectric material by adding fluorine thereto. A paper presented at the Feb. 21-22, 1995 DUMIC Conference by L. Qian et al., entitled "High Density Plasma Deposition and Deep Submicron Gap Fill with Low Dielectric Constant SiOF Films" describes deposition of up to 10 atomic % fluorine-containing moisture resistant SiOF films on a silicon sample at room temperature using high density plasma. This paper states that fluorine in the film can be reduced by adding hydrogen to the SiF.sub.4 +O.sub.2 +Ar deposition gas chemistry, the film had a dielectric constant of 3.7, and the refractive index was lowest for deposition conditions where the SiF.sub.4 :SiF.sub.4 +O.sub.2 ratio was 0.29.
Another paper presented at the DUMIC Conference is by D. Schuchmann et al., entitled "Comparison of PECVD F-TEOS Films and High Density Plasma SiOF Films." This paper mentions that fluorinated TEOS films have been used for gap filling and compares such films to films deposited by high density plasma (HDP) inductively coupled plasma using SiF.sub.4 +O.sub.2 +Ar. The HDP films were found to have better moisture and thermal stability than the F-TEOS films.
Other papers presented at the DUMIC Conference include "Preparation of SiOF Films with Low Dielectric Constant by ECR Plasma CVD" by T. Fukada et al., "An Evaluation of Fluorine Doped PETEOS on Gap Fill Ability and Film Characterization" by K. Hewes et al., "Dual Frequency Plasma CVD Fluorosilicate Glass Water Absorption and Stability" by M. Shapiro et al., and "Water-absorption mechanisms of F-doped PECVD SiO.sub.2 with Low-Dielectric Constant" by H. Miyajima et al. Of these, Fukada discloses that SiOF films deposited by rf biased ECR plasma are superior to SOG and TEOS-O.sub.3 films, the SiOF films providing excellent planarization and sub half micron gap filling without voids. Moreover, according to Fukada, the dielectric constant of SiOF films can be reduced from 4.0 to 3.2 by increasing the SiF.sub.4 /(SiF.sub.4 +SiH.sub.4) gas flow ratio in an rf-biased ECR plasma CVD process using SiF.sub.4, SiH.sub.4 and O.sub.2 gas reactants (O.sub.2 /(SiF.sub.4 +SiH.sub.4)=1.6) and a substrate held on a water cooled electrostatic chuck. Hewes discloses CVD of fluorosilicate glass films from TEOS, O.sub.2 and C.sub.2 F.sub.6 introduced into a reaction chamber by a showerhead gas mixer. Shapiro discloses that ULSI device speed can be increased by reducing capacitance of the interlevel insulator such as by adding fluorine to SiO.sub.x films but water incorporation into the films raises the dielectric constant and water evolution can produce voids or corrosion in surrounding metal. Miyajima discloses that water absorption of F-doped SiO.sub.2 films containing more than 4% F is a serious problem because it causes degradation of device reliability and film adhesion properties and that the resistance to water absorption is lower for films deposited by parallel plate plasma CVD compared to high density helicon-wave plasma using TEOS, O.sub.2 and CF.sub.4 as deposition gases.
The effects of thermal annealing on the densification of SiO.sub.2 prepared by liquid-phase deposition at 15.degree. C. is described by C. Yeh et al., in "Controlling Fluorine Concentration and Thermal Annealing Effect on Liquid-Phase Deposited SiO.sub.2-x F.sub.x Films", J. Electrochem, Vol. 142, No. 10, October 1995. Yeh discloses that restructuring occurs during annealing because H atoms between F and O atoms are very electronegative and annealing at 300.degree. to 500.degree. C. can break SiO--H bonds forming SiO.sup.- whereas annealing higher than 700.degree. C. also breaks SiF bonds forming Si.sup.+.
As integrated circuits become smaller and faster, there is a need to reduce the dielectric constant of the intermetal dielectric to prevent degradation of the pulse propagation and to reduce the device power consumption. One technique for achieving dielectric constant as low as 3.4 is by the addition of fluorine to SiO.sub.2. Fluorine doped SiO.sub.2 is commonly referred to as "FSG." FSG can be obtained in several ways such as by the addition of C.sub.2 F.sub.6 to a TEOS (tetraethoxysilane)/O.sub.2 deposition process or by plasma deposition using a mixture of SiF.sub.4 /O.sub.2 or a mixture of SiF.sub.4, SiH.sub.4 and O.sub.2.
The preferred method of FSG deposition utilizes a high density plasma source because of its demonstrated superiority in the gap filling of narrow, high aspect gaps between adjacent metal lines. One negative feature common to FSG processes is that the resulting dielectric material is hydrophilic, i.e. it absorbs moisture from the atmosphere via SiOH bonding which can result in a time dependent increase in the dielectric constant referred to as "drift." This dielectric constant drift is referred to as film instability.
As disclosed in copending application Ser. No. 08/604,018, film stability can be improved by carrying out the deposition at a process temperature greater than 300.degree. C. with SiF.sub.4 constituting more than 50% of the silicon bearing deposition gas. Although the film stability can be greatly improved, the dielectric constant can increase as much as 0.1 after one week exposure to atmospheric moisture starting from a dielectric constant of 3.5 or less. Such atmospheric exposure could occur as a result of storage time in a semiconductor fab while the wafers wait for the next processing step. For purposes of this discussion, "instability" is defined as the change in dielectric constant, .DELTA.k, when the film is left in air at ambient temperature (e.g. typically 50.+-.10% relative humidity in a fab) for 7 days. The dielectric constant is measured at least five times during the 7 day period and a least squares linear or quadratic fit to the data is used to calculate the drift at 5 days. For stability, this number is preferably less than 0.05.
Takeishi et al. investigated the film stability effects of annealing an oxide film in N.sub.2 O, N.sub.2 and O.sub.2 (J. Electrochem. Soc., 143,381 (1996). According to Takeishi et al., the film was heated to 400.degree. C. in a N.sub.2 O plasma, an N.sub.2 plasma, an O.sub.2 plasma and O.sub.2 without plasma. The film, deposited from plasma enhanced CVD TEOS/O.sub.2 using C.sub.2 F.sub.6 as the fluorine source, showed some improvement in stability for the nitrogen and N.sub.2 O treated films. A major drawback of the Takeishi et al. treatment is that a 30 min. plasma treatment is necessary to achieve stability improvement. Such a lengthy treatment presents serious problems in implementation as a production process using single wafer processing technology.
Hattangady et al. investigated a process for nitriding the surface of a gate oxide by exposing the oxide to a remotely generated He--N.sub.2 plasma. According to Hattangady et al., a surface nitridation of an SiO.sub.2 film increases with increasing temperature and saturates with a sufficiently long exposure time on the order of about 50 minutes.